Data link communication systems transmit messages between aircraft and ground stations via radio or satellite. A network of ground radio stations ensure that aircraft can communicate with ground in real-time from practically anywhere in the world. Satellites are used over oceans or remote areas were no ground stations exist. Aircraft communications addressing and reporting system (ACARS) is a data link system that handles text-based information of essentially the same type as can be sent via ground-ground telex. A person or a system on board creates a message and sends it via ACARS to a system or user on the ground and vice versa. Messages are sent both automatically and manually.
There are three major components to the ACARS data link system: aircraft equipment, service providers, and ground processing systems. An ACARS communications management unit (CMU or MU) is on board the aircraft. The MU is connected to a number of other devices on board the airplane: a very high frequency (VHF) radio, a keyboard and display for the pilot master control display unit (MCDU) and a printer. In some cases, the CMU is also connected to other systems.
The data link service provider delivers a message from the aircraft to the ground station, and vice versa. The data link service provider operates a network of ACARS VHF remote ground stations (RGSs). Service providers also provide service via SATCOM and HF data link as an alternative to VHF to provide full data link capability also in remote areas or over oceans.
The ground processing system, such as Honeywell's global data centre (GDC), performs all data link-specific tasks, maintains connection with service providers, logs messages, etc. The ground processing system's data link application, such as, weather information and flight planning engine, is connected to back-end computer systems at the ground processing system. The ground processing system and ACARS together provide real-time communication between the ground and aircraft.
There are technical limitations of currently available data link communications. The ACARS data link is limited by a low-speed air/ground VHF link. Messages must be kept short, since the delivery performance decreases exponentially with message size. The maximum block size of the data link message for the ACARS data link is 220 characters or less. The average real-life performance has an end-to-end delivery time of 10-20 seconds in the uplink message, and 5-10 seconds in the downlink message. In the case of multi-block uplinks, the system experiences high latency.
Current AirSat II Iridium satellite communication systems are designed to establish connection to ACARS communications management units allowing the transfer of information to and from the ground processing system, such as Honeywell Global Data Center, using the standard Airborne Flight Information System (AFIS) protocol. The AFIS protocol has a limit on message size that can be datalinked.